Instead of that try watering the plants less or let their soil dry out a bit. The flies are probably the end result of maggots feeding on decaying material or growing fungus in the pot soil. That is my assumption since you didn't mention any pest seen directly on the plants themselves.

http://www.gardenersworld.com/how-to/projects/greenhouse/how-to-tackle-house-plant-flies/4149.html

Lots of info here should help

I have these little flies too! Thanks for the link Steragram, top tips!

To add to the link, products containing the mosquito control Bacillus thuringiensis israelensis have also proven effective. I have also used weak tea to good effect.

I had little flies in my soil, I used fly spray around the plants & Wow! The table ended up with loads of tiny dead ones, I had to hoover them off but they kept coming, I had used Garden Compost instead of Houseplant Compost & that was the reason I had flies in the soil so I repotted the plants in houseplant compost & now all is ok. Good Luck with yours :o}}

hi what i do is get a mineral plastic bottle cut it a third from the top, put a banana skin in it, now tape a cone of cardboard around the open neck ,cutting a hole a little bigger than a pin head, invert it into the big end of the bottle ,then seal it thoroughly with cello tape he presto a trap, flies are attracted by rotten gases.

That's very clever Eamonn. Perhaps you should do a blog about it with pictures?

Works great with fruit flies. Used the same trap to to coral the strays that got loose in the lab. Give it a try.